In a contrast correction technique of the related art, when correction is performed to improve visibility of a part of luminance regions for an image having a wide luminance range from a low luminance region to a high luminance region, a gradation collapse occurs in any one of the luminance regions excluded from a target in some cases. For example, when taking a picture containing dark indoor and bright outdoor images at the same time from the bright outdoor place, and trying to raise visibility of the subject reflected in the indoor place by contrast correction, the luminance region of the outdoor image is collapsed.
In this regard, in order to address such a problem, there is known a technique of locally correcting contrast of the image. For example, a retinex theory is known in the art. The retinex theory is a theory for explaining human visual characteristics called color constancy or lightness constancy. In accordance with this theory, a reflection light component can be extracted by separating an illumination light component from the image. Therefore, in the image correction processing based on the retinex theory, it is possible to obtain a high visibility image by removing influence of the illumination light component which makes it difficult to view a subject in an image containing a dark indoor place or under bright backlight and extracting the reflection light component. For this reason, it is possible to desirably compress a natural dynamic range perceptible by human beings even in a digital grayscale.
As an example of the image processing techniques based on the retinex theory, for example, Patent Document 1 discusses multi-scale retinex processing. In Patent Document 1, any one of blurred images having different blurring degrees generated from a plurality of ambient functions having different scales is selected depending on a pixel value level of a processing target original image. As a result, synthesis blurring is created, and lowpass filtering is applied to the created image, so that retinex processing capable of preventing occurrence of unnatural boundary discontinuity is performed. Note that the multi-scale retinex processing will be described below in more details.
As an image correction technique based on the retinex theory and the Phong reflection model, Patent Document 2 is known in the art. The image processing apparatus discussed in Patent Document 2 performs high-quality image correction processing by extracting a specular reflection component and a diffuse reflection component from an input image signal and generating a correction image signal on the basis of a computation result for the extracted specular reflection component and the extracted diffuse reflection component. Note that the Phong reflection model will be described below in more details.